The field of the disclosure relates generally to a drive circuit for an electric motor, specifically a permanent split capacitor (PSC) motor and, more specifically, a drive circuit that enables synchronized transfer to line frequency power under load.
At least some known PSC motors are fixed speed motors that operate most efficiently at line frequency power. Such PSC motors exhibit uncontrolled acceleration during startup. Further, at low load conditions, such PSC motors operate less efficiently. Alternatively, a PSC motor may be driven with a variable speed motor controller to adapt motor speed to a load level. Such configurations are generally limited by power factor, electromagnetic interference, and electrical losses.
A hybrid drive circuit for PSC motors enables efficient operation at both high and low load conditions. For example, a PSC motor operating a compressor in a heating, ventilation and air conditioning (HVAC) system may experience high load conditions during peak temperatures and low load conditions during milder temperatures. The drive circuit operates the PSC motor using an inverter under low load conditions, and operates the PSC motor using line frequency power under high load conditions.
When starting up a compressor, the load on the PSC motor is generally low and builds over time as suction and discharge pressures increase the torque demand on the PSC motor. The starting torque output of the PSC motor, at line frequency power, for example, is generally higher than the torque load at startup. Conversely, when a compressor has been operating for some time, suction and discharge pressures may build up that produce a torque load that exceeds the starting torque output, thus preventing the PSC motor from turning, i.e., a locked rotor or a stalled compressor. At least some system controllers for PSC motors include an interlock that prevents restarts of the PSC motor until pressures have equalized in the compressor, thereby relieving the starting torque load. Such interlocks may be on the order of minutes in duration, during which the compressor cannot operate.
More specifically, in operating a compressor, the inverter and line frequency power cannot both be connected to the electric motor at the same time, because of the potential for a line-to-line short circuit. To transition from inverter to line, or line to inverter, one is disconnected before connecting the other. However, while the inverter can be disconnected in microseconds, the contactor regulating line frequency power to the electric motor can require up to two line cycles, or approximately 16-32 milliseconds (ms), to open or close. Consequently, during transitions from inverter to line or line to inverter, current through the electric motor may decay to zero, leading to a motor stop. Motor speed may decay below a threshold speed within a single line cycle. In some electric motors, the threshold speed may be zero, while for other electric motors, the threshold speed may be above zero. When transitioning from line frequency power to the inverter, the starting torque output available through the inverter generally exceeds the load torque on the electric motor and is limited typically only by the current ratings of the switching components of the inverter. Speed decay is not a problem during such a transition. However, when transitioning from the inverter to line frequency power, the starting torque output at line frequency power may fall below the torque demand from the compressor. Under such conditions, the compressor can stall, i.e., winding current, motor speed, and motor torque decay within a single line cycle, or approximately 16 ms. The typical interlock duration for restarting an electric motor operating a compressor is too long for the electric motor to transition between operation with the inverter to operation at line-frequency power for effective system operation.
Generally, replacing the contactor regulating line frequency power to the electric motor with faster solid state switches is cost-prohibitive in terms of efficiency and thermal management. For example, a typical two pole contactor might consume 12 watts of power, while a suitable solid state substitute, such as, for example, a triode for alternating current (TRIAC), would consume 50 watts.
Solid state switches coupled between the line frequency power source and the stator windings, and wired in parallel to the contactor poles for transitioning between the inverter to line frequency power enable seamless transition without the disadvantages of complete replacement of the contactor. The solid state switches may be limited to conduction for the several line cycles until the contactor closes. Due to rotor slip when the electric motor accelerates to line frequency, the motor current demanded may exceed normal full load motor current demand by up to an order of magnitude for a brief period of time. Accordingly, the solid state switches are capable of conducting peak currents for several line cycles at higher levels than normal operating current. Such solid state switches may include, for example, TRIACs. The solid state switches can close within 3 ms and, in certain embodiments, within 1 ms and, notably, before winding currents decay below a current threshold, which avoids the potential for the compressor to stall during the transition from the inverter to line frequency power. Such a current threshold is generally defined by the torque required to turn the motor given a certain load and motor speed. For example, in certain embodiments, the current threshold may range from zero to 60 amperes depending on motor load and motor speed.
Drive circuits for electric motors are subject to various transient electrical events that can damage the drive circuit or the electric motor itself if not handled properly. Transient electrical events are generally characterized by a rapid change in potential, or voltage, over a brief period of time, otherwise referred to as high delta-Voltage/delta-time, or high dV/dt, events. Such events most often occur on alternating current (AC) lines due to lightning strikes, but may also result from poor or loose connections. Likewise, high dV/dt events may also result from a potential buildup between a chassis, or casing, of the electric motor and the stator windings, which itself may result from a lightning strike, for example. The potential buildup may then be capacitively coupled into the drive circuit through the stator windings. In addition, normal operation of the drive circuit and, more specifically, the switching executed within the inverter, can create high dV/dt noise that can damage components of the drive circuit under certain conditions. For example, under nominal ambient conditions, the high dV/dt noise may be handled properly and not rise to the threshold of damaging the drive circuit. However, under sub-optimal ambient conditions such as, for example, elevated ambient temperatures or humidity, the high dV/dt noise may result in a transient event that can cause damage to the drive circuit. Drive circuits, or “hybrid drive circuits,” that include a line synchronization circuit are particularly susceptible to transient electrical, or high dV/dt, events, such as ring waves and combination waves.